Silica gels and other silica components are widely used as catalyst supports in industry. For fixed bed type processes, silica-supported catalysts in a formed shape, such as beads or extrudates, are required. Two approaches have been commonly used to produce formed silica-supported catalyst particles. One involves the impregnation of catalyst components on preformed silica particles, and the other involves the preparation of silica-supported catalyst powders and then processing the powders into formed catalyst particles.
The formation of silica gel beads is a common practice in industry and well documented in open literature. Although silica gel beads with good mechanical strength can be obtained, these beads are (in general) susceptable to cracking when they are immersed in water. This precludes the use of direct aqueous impregnation on silica xerogel beads as a way to produce silica-supported catalyst beads.
To alleviate this cracking/breakage problem of silica gel beads, two types of methods have been developed. One uses organic solvents rather than water as the impregnation medium. The much lower surface tension of organic solvents significantly reduces the cracking and breakage of silica gel beads. However, the use of organic solvents increases not only the cost of manufacturing but also the complexity of the process. Another type of method aims to improve the mechanical strength of silica gel beads by calcining the beads at high temperatures (800-1000° C.), prior to catalyst impregnation. Calcining silica gels at high temperatures, however, increases the manufacturing cost and changes the surface chemistry of the silica gel, particularly causing the sintering of silica gel and the reduction of surface hydroxyl concentration. This may be very undesirable because hydroxyl groups may be the anchoring sites for many catalytic components. In both types of methods, two drying steps, one for the drying of silica gel beads and the other for the drying of catalyst beads, are necessary.
In many processes, more than one drying step is contemplated. Drying processes are typically costly. Therefore, it would be desirable to develop a process for making high strength catalysts which minimizes the number of drying steps required.